From studies using highly purified or homogeneous preparations of rabbit reticulocyte initiation factors, it appears that the mechanism of initiation complex formation is the same in both A. salina (brine shrimp) and rabbit reticulocytes. Further, the high degree of replacement of A. salina components by the more purified reticulocyte initiation factors (in an assay system using A. salina ribosomes) indicates that despite the wide evolutionary gulf between A. salina and rabbits, there appears to have been a restricted divergence of the more than 15 proteins (polypeptide chains) which are required for initiation complex formation.